During the manufacture of fibers by melt-spinning a polymer through an extruder with a spinnerette the melt is filtered at the spinnerette to provide a homogeneous polymer to the spinnerette and to remove melt impurities like gels, carbonized materials and inorganic particles. Ineffective filtration increases the incidence of fiber breakage during spinning and drawing and clogged spinnerettes, J. Cornelsen, Oelde, "New Developments in the Filtration of Polymer Melts," Chemiefeslon/Tesetil-industrie, Sol. 34/86, June 1984, pp. 409-411.
Two different types of filter elements are used in polymer spin pack filtration to remove the contaminants mentioned above. These are surface and depth elements, Engineering Report No. 1, Fugi Filter Manufacturing Co. LTD.
Surface elements capture all particles by surface disposition, requiring all the pores of the element to be about the same size to achieve the desired degree of filtration. A single layer of wire screen mesh is an example of this type of media.
The depth filter element is made up of a porous structure with thickness and voids which allow particulate matter to be trapped in the interstices and the clean polymer to flow through. This "straining in depth" action of a depth filter is enhanced by changes in the fluid velocity and direction through the porous structure, which results in entrapment and sedimentation of the particles.
Examples of depth filter elements are wire cloth, random wire fiber, and sintered metal powders. The first type of filters can be easily cleaned because the particles are retained on the surface of the filter.
Cleaning methods for the second filter type are more difficult, and typically involve cleaning with chemical and thermal means with ultrasonics as a post-cleaning step.
Cleaning by thermal means are described for example in Duffy, Robert E., "Basic Methods for Removing Polymer from Parts," International Fiber Journal, June 1991.
Methods comprise incinerating furnace or oven, salt bath, vacuum oven and fluidized bed.
Chemical methods are described for example in Fourne, F., "Cleaning of Polymer Soiled Machine Parts," Chemie-fasern/Textilindustrie (CTI), Vol. 35/87 (June 1985), pp. 397-403, and comprise use of solvents.
Postcleaning operations like ultrasonic cleaner are described for example in Fourne F., "Cleaning of Machine Parts and Molds Contaminated With Plastics," Kunststoffe, 79 (1989) 9, pp. 807-813.
Disadvantage of all methods of the prior art is that the cleaning is insufficient and rest particles stay in the filter causing a faster pressure build up when reused.
Object of the present invention was to provide a method of cleaning depth filter elements, especially sintered metal filters so that the pressure build up after the cleaning is as low as possible and the air permeability through the filter does not derive significantly in comparison to a new filter.